(1) Field of the Invention
This invention relates to an improved delayed coking process using antifoam additives to reduce foaming in the process.
(2) General Background
Delayed coking operations in most modern refineries produce solid coke, gaseous and liquid products from heavy residual oil feedstocks. In the usual application of this process, a heavy residual oil is heated in a furnace, passed through a transfer line and discharged into the bottom of a vertically positioned coke drum. A certain amount of steam accompanies the feed passing into the coke drum to maintain appropriate velocity through the furnace and the transfer line to help reduce fouling on these hot lines.
In the coke drum, the residual feedstock is thermally decomposed to a very heavy tar or pitch which further decomposes into solid coke and vapor materials. The vapors are recovered from the delayed coking drum, and a solid coke material is left behind. After a period, the feed to the coke drum is stopped and routed to another drum where the coking continues. The original coke drum is then purged, cooled and opened so that solid coke inside can be removed.
The vapor products removed from the coke drum are passed to fractionation means, and the recovered liquids pass to other processing units in the refinery. These units include hydrotreaters, desulfurizers, reformers, hydrocrackers or fluidized catalytic cracking units which upgrade the liquids.
During normal delayed coking operations, quantities of foam are produced in the coke drum. This is undesirable since the foam diminishes the efficient operations of the process. During the latter stages of the coke-producing cycle, when the coke drum becomes full of solid coke, the foam which is accumulating at the top of the coke bed can eventually be entrained with vapors leaving the coke drum and be carried over into the fractionation section
To control the foam in the delayed coking process, it is customary to treat the materials within the coke drum during the coking cycle with antifoam agents. The typical antifoam agents are the silicone-containing polymers which typically have viscosities, when measured by conventional known methods, varying anywhere from about 12,000 up to about 300,000 centistokes. The preferred silicone polymer is polydimethyl siloxane.
Addition of the siloxane polymer reduces the surface tension of the foam in the coke drum causing it to break, thereby reducing or eliminating its presence. The antifoam agents are added during the last few hours of the delayed coking cycle--at a point when the foam level in the coke drum is approaching the upper portion of the coke drum.
The antifoam materials are generally added to the upper portion of the coke drum either through its side wall or through the head of the coke drum. Normally, the antifoam agents are circulated externally through a closed loop and injected into the upper portion of the coke drum at an appropriate time.
In order to prevent coking and eventual plugging of the lines that carry the antifoam agents into the coke drum, they are normally purged with steam because near the coke drum their wall temperatures can be as high as 800 to 900.degree. F. depending upon the refiner's operations at the coker. A typical and inexpensive purge that refiners use is steam, which allows enough velocity in these lines to prevent plugging through deposition of coke. When anti-foam materials are to be injected into the coke drum, a valve will open and inject the circulating antifoam agent along with the steam purging material into the coke drum where it can contact the foam and reduce or eliminate its presence.
One of the problems associated with using steam as a purge is that it adversely affects stability of the polysiloxane polymer causing it to break down or decompose into more volatile silicon-containing materials which are carried away with the vapors leaving the coke drum. This silicon can contaminate downstream processes which are fed the liquids recovered from the coke drum.
Silica poisoning of these processes is expensive to the refiner, since product quality can suffer. To overcome the poisoning generally associated with silica, catalyst must often be regenerated or replaced.
U.S. Pat. No. 3,700,587, issued Mar. 1, 1971 Class 208-131 discloses in general the use of silicon antifoam material for use in delayed coking process units.
U.S. Pat. No. 4,404,092, issued Sept. 13, 1983 relates to a general delayed coking process.
The following article discloses the use of silicone materials for defoaming various petroleum processes including delayed cokers: Pape, P. G., "Silicones: Unique Chemicals for Petroleum Processing," J. Pet. Proc. Inc., June 83, pp 1197-1204.